Hi,
sabujp wrote on 2008-08-04 23:18:41 -0400 [[BackupPC-users] clustered file
system and multiple servers]:
>
> Can a dev let me know if the files in the pool are FLOCK'd before writing,
use the force, read the source. "grep -r flock backuppc-3.1.0" suggests that
flock is used, but not on pool files. Not surprising, considering BackupPC_link
and BackupPC_nightly (or two instances of BackupPC_link) may not run
concurrently (BackupPC_link is responsible for entering new files into the
pool).
Come to think of it, the reason for this restriction is of a different nature:
BackupPC_nightly sometimes needs to rename pool files (with a common BackupPC
hash, when one or more files out of the chain are deleted), while BackupPC_link
may insert a new file with the same BackupPC hash. You can't prevent the
resulting race condition with flock() - at least you don't effectively change
anything in the single-threaded case (you'd need a rather global lock).
> i.e. is there a chance that two servers backing up into the same top level
> data directory could "mangle" a file in the pool in this manner?
I don't think you'd have mixed file contents (or, effectively, a corrupt
compressed file), but there seems to be a chance of linking a file in a backup
to the wrong pool file (wrong contents altogether).
You could probably use flock() to prevent two instances of
BackupPC_link/BackupPC_nightly running simultaneously on different servers,
but there are more things you would want to think about (running
BackupPC_nightly on more than one server does not make much sense, even if
they don't run concurrently; limiting simultaneous backups over all servers;
ensuring BackupPC_dump is not run twice simultaneously for the same host ...).
In short: sharing a pool between servers is currently not supported.
Regards,
Holger
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